We found a significantly higher rate of immediate progression in rectal NEC with ARID1A deletions (56 vs 33%, OR 6.43, 95%CI 1.05–39.33, p = 0.044) and a strong trend towards inferior RR (OR 0.19, 95%CI 0.03–1.02, p = 0.052). The gene discussed is ARID1A; the disease is neuroendocrine carcinoma.